1. Description of the Related Art
U.S. Pat. No. 5,178,215 serves as a starting point for the departure made by the present invention. The disclosure of U.S. Pat. No. 5,178,215 is incorporated herein by reference and includes a general discussion of an existing rotary blowout preventer which is fluid actuated to grip a drill pipe or kelly, and the controlled circulation of a fluid to lubricate and cool bearings and seals, and to filter particulate matter.
These existing rotary blowout preventers have an annulus between an outer housing and a rotary housing. Such systems use rather large bearings which require a rather large clearance. Such an arrangement has positive effects but also results in “wobbling” between the rotary housing and the outer housing. The wobbling creates heat, “nibbles” the seals, etc. A fluid is introduced into and circulates through the annulus between the outer housing and the rotary housing to cool the seal assemblies, the bearings and to counteract heat generated by contact between the seals and the rotary housing (wellhead fluid temperatures may normally be about 200° F., and during rotation, without cooling, the temperature would readily increase to about 350° F. and destroy a seal in a relatively short time). The circulated fluid also removes foreign particulate matter from the system. Pumps are used to maintain a fluid pressure in the annulus at a selected pressure differential above the well bore pressure.
The bearings in these rotary blowout preventers may normally operate at a temperature of about 250° F. Such bearings are subjected to a significant thrust load, e.g. 2,000 lbs.-force, due in part to an upward force created by well bore pressures and placed upon a packer assembly and a sleeve in the rotary housing. Such a thrust load will generate significant heat in a bearing rotating at, for example, 200 rpm. Heat, and heat over time, are important factors which may lead to bearing failure. For example, bearings may immediately fail if they reach temperatures of about 550° F. Even at temperatures of 250° F. a bearing may fail after a significant period of use, for example, twenty days of rotation at 200 rpm when subjected to a significant thrust load.
Such existing rotary blowout preventers are very functional at wellhead pressures up to 2000 psi. However, for reasons discussed herein, there are added challenges when wellhead pressures are in the range of, for example, 2500 psi to 5000 psi.
For example, as suggested, the continued and trouble free operability of such rotary blowout preventers is dependent, in part, upon the life of the seals and bearings within the rotary blowout preventer. The seals have a “pressure/velocity” or “pv” rating which may be used to predict the relative life of a seal given the pressure and velocity conditions to be borne by a seal. When considering “PV” rating, it is significant to note that a linear relationship does not exist between the life of a seal and the increases in pressure or rotational velocity to which a seal will be subjected. Rather, the life of the seal decreases exponentially as the pressure or rotational velocity to which the seal is subjected is increased.
As such, when well bore pressures increase to ranges from 2500 psi to 5000 psi, the loads, the wear and the heat exerted on seals and bearings within a rotary blowout preventer pose a greater challenge to the operations and life of the seals and bearings. This must be considered in the context of the fact that well bore operations may be shut down for maintenance work when significant wear of seals or bearings, significant “nibbling” of seals, or seal/bearing failure occurs. Such shut downs can significantly affect the profitability of well bore operations.